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  • Journal article
    Prosser R, Patel Y, Offer GJ, 2020,

    Lithium-Ion Battery Degradation Mode Diagnostics Using Heat Generation Profiles

    , ECS Meeting Abstracts, Vol: MA2020-02, Pages: 3175-3175

    <jats:p> As lithium ion cells are used, internal chemical and physical degradation processes occur which negatively impact cell performance. A key issue for battery systems engineers is extending the life of their battery pack which amounts to slowing down the rate of these degradation processes.</jats:p> <jats:p>The first step in addressing this problem is being able to frequently and accurately diagnose cell degradation. With this knowledge of cell state, it is possible to make better decisions on how to adjust the operating conditions of the battery pack to optimise cell longevity and performance.</jats:p> <jats:p>Current methods which are capable of accurate quantitative diagnostics employ thermodynamic models parameterised by different degradation modes. These models require very slow discharge rates or galvanic intermittent titration tests (GITT) to obtain the cell open circuit voltage (OCV) and therefore cannot be used in most commercial applications. Other methods which do not require OCV conditions involve an impedance model coupled to the thermodynamic model. This adds computational complexity of the methods as well as incurring an accuracy penalty compared to the thermodynamic models.</jats:p> <jats:p>We present a novel technique which uses cell instantaneous heat generation rate obtained in operando to quantify the extent of cell degradation. The proposed method provides results comparable to those obtained from thermodynamic models using a model which is no more computationally intense. The method also decouples the thermodynamic and kinetic effects of degradation allowing for a full diagnosis to be obtained more accurately and in a fraction of the time compared to alternative methods.</jats:p> <jats:p>With this powerful and simple method, a battery management system would be able to make better and more frequent adjustments to its cell’

  • Conference paper
    Schimpe M, Barreras JV, Wu B, Offer GJet al., 2020,

    Novel Degradation Model-Based Current Derating Strategy for Lithium-Ion-Batteries

    , Publisher: The Electrochemical Society, Pages: 3808-3808

    <jats:p> Derating is the operation of an electrical or electronic device at less than its rated maximum capability in order to ensure safety, extend lifetime or avoid system shutdown. Relatively simple derating approaches have been proven effective for lithium-ion batteries. They are typically based on limiting battery charging and discharging currents to prevent operation outside certain operating areas, which are bounded by state-of-charge (SOC), voltage, or temperature levels, taken individually. The manufacturer’s datasheet provides hard limits for these operating areas, defining the so-called safe operating area (SOA). In order to prolong battery lifetime, more restrictive limits than the SOA can be defined, but this leads to reducing battery performance more frequently and intensively. However, it should be noted that these simple derating approaches do not fully capture the complexity of battery degradation mechanisms, since the actual rate of degradation is the result of an interaction of multiple operating conditions. Thus, they may overestimate or underestimate the optimal current limit. Indeed, many advanced degradation models that consider a combination of operating conditions have been proposed in the literature to predict the rate of degradation, in terms of capacity loss and/or internal resistance increase.</jats:p> <jats:p>With this in mind, we propose the integration of an advanced degradation model in the derating strategy and thereby reduce degradation without significant losses in performance. The degradation model calculates the maximum battery current that will ensure reduced degradation rates, both for calendar and cycle related ageing processes. The calendar ageing rate is limited by defining the SOC-dependent maximum temperature that will keep the rate below a certain level, and then limiting the current accordingly, aiming to reduce self-heating effects that lead to temperature rise. The cycle ageing

  • Journal article
    Hales A, Prosser R, Diaz LB, White G, Patel Y, Offer Get al., 2020,

    The Cell Cooling Coefficient as a design tool to optimise thermal management of lithium-ion cells in battery packs

    , ETRANSPORTATION, Vol: 6, ISSN: 2590-1168
  • Journal article
    Chen B, Zhang H, Xuan J, Offer GJ, Wang Het al., 2020,

    Seeing is Believing: In Situ/Operando Optical Microscopy for Probing Electrochemical Energy Systems

    , ADVANCED MATERIALS TECHNOLOGIES, Vol: 5, ISSN: 2365-709X
  • Journal article
    Jiang Y, Offer GJ, Jiang J, Marinescu M, Wang Het al., 2020,

    Voltage hysteresis model for silicon electrodes for lithium ion batteries, including multi-step phase transformations, crystallization and amorphization

    , Journal of the Electrochemical Society, Vol: 167, Pages: 1-9, ISSN: 0013-4651

    Silicon has been an attractive alternative to graphite as an anode material in lithium-ion batteries (LIBs). The development of better silicon electrodes and the optimization of their operating conditions for longer cycle life require a quantitative understanding of the lithiation/delithiation mechanisms of silicon and how they are linked to the electrode behaviors. Herein we present a zero-dimensional mechanistic model of silicon anodes in LIBs. The model, for the first time, quantitatively accounts for the multi-step phase transformations, crystallization and amorphization of different lithium-silicon phases during cycling while being able to capture the electrode behaviors under different lithiation depths. Based on the model, a linkage between the underlying reaction processes and electrochemical performance is established. In particular, the two sloping voltage plateaus at low lithiation depth are correlated with two electrochemical phase transformations and the emergence of the single broad plateau at high lithiation depth is correlated with the amorphization of c-Li15Si4. The model is then used to study the effects of crystallization rate and surface energy barriers, which clarifies the role of surface energy and particle size in determining the performance behaviors of silicon. The model is a necessary tool for future design and development of high-energy-density, longer-life silicon-based LIBs.

  • Journal article
    Bravo Diaz L, He X, Hu Z, Restuccia F, Marinescu M, Barreras JV, Patel Y, Offer G, Rein Get al., 2020,

    Review—meta-review of fire safety of lithium-ion batteries: industry challenges and research contributions

    , Journal of The Electrochemical Society, Vol: 167, Pages: 1-14, ISSN: 0013-4651

    The Lithium-ion battery (LIB) is an important technology for the present and future of energy storage, transport, and consumer electronics. However, many LIB types display a tendency to ignite or release gases. Although statistically rare, LIB fires pose hazards which are significantly different to other fire hazards in terms of initiation route, rate of spread, duration, toxicity, and suppression. For the first time, this paper collects and analyses the safety challenges faced by LIB industries across sectors, and compares them to the research contributions found in all the review papers in the field. The comparison identifies knowledge gaps and opportunities going forward. Industry and research efforts agree on the importance of understanding thermal runaway at the component and cell scales, and on the importance of developing prevention technologies. But much less research attention has been given to safety at the module and pack scales, or to other fire protection layers, such as compartmentation, detection or suppression. In order to close the gaps found and accelerate the arrival of new LIB safety solutions, we recommend closer collaborations between the battery and fire safety communities, which, supported by the major industries, could drive improvements, integration and harmonization of LIB safety across sectors.

  • Journal article
    Dondelewski O, Szemberg OConnor T, Zhao Y, Hunt IA, Holland A, Hales A, Offer GJ, Patel Yet al., 2020,

    The role of cell geometry when selecting tab or surface cooling to minimise cell degradation

    , eTransportation, Vol: 5, Pages: 1-12, ISSN: 2590-1168

    Thermal management of lithium ion batteries is critical to maintain cells at their optimum temperature and balance performance with degradation. Previous work has shown tab cooling to be better for performance and lifetime, but only if sufficient heat removal can be achieved, which depends in part on cell geometry. In this paper, a large form-factor pouch cell is shown to suffer from faster degradation when tab-cooled although still benefitting from higher useable energy. This paper introduces the ratio of surface-to-tab cell cooling coefficient, CCCratio, as a qualitative measure to assess a cell’s suitability for tab cooling. For low CCCratio cells, tab cooling results in more useable energy and lower degradation rates than surface cooling. However, the large pouch cell used in this study has a high CCCratio, indicating that it is difficult to remove sufficient heat through tab cooling. At beginning of life, tab cooling allows access to more usable energy in the cell, but the rate of high temperature-induced degradation is greater, compared to the surface cooled cell. As a result, the useable energy from the tab cooled cell diminishes more rapidly, and after a certain cycle count, the useable energy from the surface cooled cell is superior. The optimum cooling approach will therefore be dependent on the desired lifetime of the system. This research should be of particular interest to cell and battery pack designers.

  • Report
    Offer G, Szemberg O'Connor T, De Marco M, 2020,

    Opportunities for disruptive advances through engineering for next generation energy storage

    Throughout human history, major economic disruption has been due to technological breakthroughs.Since 1990 the energy density of lithium-ion cells has increased by a factor of four and the cost has dropped by a factor of 10.This has caused disruption to the energy industry, but advances are slowing.The manufacturing and supply chain complexity means that the next big technology will take 15 years to dominate.The academic literature charts this process of development and can be used to show what is in the pipeline.Three candidates that have had a large increase in publication count are: lithium sulphur, solid-state, and sodium-ion technology.From the level of investments in start-ups and academic publication counts, solid‑state cells are closest to maturity.To identify disruption potential, look at uncertainty in performance. Cell lifetime in lithium-ion cells indicates room for improvement.Define a new disruption metric: . Look for areas of industry that lower this metric.Thermal management is a lucrative area for improvement. Cooling the cell tabs of a 5Ah cell reduces the lifetime cost by 66%, compared to 8%/pa for 13 years relying on cost reduction.Second life applications lower the lifetime cost by using the remaining 75% of energy throughput available in a cell after use in an electric vehicle.Drop-in changes to standard manufacturing processes enable huge disruption. Electrolyte additives can increase cell life by 10 times, lowering lifetime cost by 90% in a simple manufacturing intervention.

  • Journal article
    Offer G, Patel Y, Hales A, Bravo Diaz L, Marzook Met al., 2020,

    Cool metric for lithium-ion batteries could spur progress

    , Nature, Vol: 582, Pages: 485-487, ISSN: 0028-0836
  • Journal article
    O'Kane SEJ, Campbell ID, Marzook MWJ, Offer GJ, Marinescu Met al., 2020,

    Physical origin of the differential voltage minimum associated with lithium plating in Li-Ion batteries

    , Journal of The Electrochemical Society, Vol: 167, Pages: 1-11, ISSN: 0013-4651

    The main barrier to fast charging of Li-ion batteries at low temperatures is the risk of short-circuiting due to lithium plating. In-situ detection of Li plating is highly sought after in order to develop fast charging strategies that avoid plating. It is widely believed that Li plating after a single fast charge can be detected and quantified by using a minimum in the differential voltage (DV) signal during the subsequent discharge, which indicates how much lithium has been stripped. In this work, a pseudo-2D physics-based model is used to investigate the effect on Li plating and stripping of concentration-dependent diffusion coefficients in the active electrode materials. A new modelling protocol is also proposed, in order to distinguish the effects of fast charging, slow charging and Li plating/stripping. The model predicts that the DV minimum associated with Li stripping is in fact a shifted and more abrupt version of a minimum caused by the stage II-stage III transition in the graphite negative electrode. Therefore, the minimum cannot be used to quantify stripping. Using concentration-dependent diffusion coefficients yields qualitatively different results to previous work. This knowledge casts doubt on the utility of DV analysis for detecting Li plating.

  • Journal article
    Madabattula G, Wu B, Marinescu M, Offer Get al., 2020,

    Degradation diagnostics for Li4Ti5O12-based lithium ion capacitors: insights from a physics-based model

    , Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

    Lithium ion capacitors are an important energy storage technology, providing the optimum combination of power, energy and cycle life for high power applications. However, there has been minimal work on understanding how they degrade and how this should influence their design. In this work, a 1D electrochemical model of a lithium ion capacitor with activated carbon (AC) as the positive electrode and lithium titanium oxide (LTO) as the negative electrode is used to simulate the consequences of different degradation mechanisms in order to explore how the capacity ratio of the two electrodes affects degradation. The model is used to identify and differentiate capacity loss due to loss of active material (LAM) in the lithiated and de-lithiated state and loss of lithium inventory (LLI). The model shows that, with lower capacity ratios (AC/LTO), LAM in the de-lithiated state cannot be identified as the excess LTO in the cell balances the capacity loss. Cells with balanced electrode capacity ratios are therefore necessary to differentiate LAM in lithiated and de-lithiated states and LLI from each other. We also propose in situ diagnostic techniques which will be useful to optimize a LIC's design. The model, built in COMSOL, is available online.

  • Journal article
    Feng X, Merla Y, Weng C, Ouyang M, He X, Liaw BY, Santhanagopalan S, Li X, Liu P, Lu L, Han X, Ren D, Wang Y, Li R, Jin C, Huang P, Yi M, Wang L, Zhao Y, Patel Y, Offer Get al., 2020,

    A reliable approach of differentiating discrete sampled-data for battery diagnosis

    , ETRANSPORTATION, Vol: 3, ISSN: 2590-1168
  • Journal article
    Hales A, Marzook MW, Bravo Diaz L, Patel Y, Offer Get al., 2020,

    The surface cell cooling coefficient: a standard to define heat rejection from lithium ion battery pouch cells

    , Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

    There is no universal and quantifiable standard to compare a given cell model's capability to reject heat. The consequence of this is suboptimal cell designs because cell manufacturers do not have a metric to optimise. The Cell Cooling Coefficient for pouch cell tab cooling (CCC tabs ) defines a cell's capability to reject heat from its tabs. However, surface cooling remains the thermal management approach of choice for automotive and other high-power applications. This study introduces a surface Cell Cooling Coefficient, CCC surf which is shown to be a fundamental property of a lithium-ion cell. CCC surf is found to be considerably larger than CCC tabs , and this is a trend anticipated for every pouch cell currently commercially available. However, surface cooling induces layer-to-layer nonuniformity which is strongly linked to reduced cell performance and reduced cell lifetime. Thus, the Cell Cooling Coefficient enables quantitative comparison of each cooling method. Further, a method is presented for using the Cell Cooling Coefficients to inform the optimal design of a battery pack thermal management system. In this manner, implementation of the Cell Cooling Coefficient can transform the industry, by minimising the requirement for computationally expensive modelling or time consuming experiments in the early stages of battery-pack design.

  • Journal article
    Yang K, Jia L, Liu X, Wang Z, Wang Y, Li Y, Chen H, Wu B, Yang L, Pan Fet al., 2020,

    Revealing the anion intercalation behavior and surface evolution of graphite in dual-ion batteries via in situ AFM

    , Nano Research, Vol: 13, Pages: 412-418, ISSN: 1998-0124

    Graphite as a positive electrode material of dual ion batteries (DIBs) has attracted tremendous attentions for its advantages including low lost, high working voltage and high energy density. However, very few literatures regarding to the real-time observation of anion intercalation behavior and surface evolution of graphite in DIBs have been reported. Herein, we use in situ atomic force microscope (AFM) to directly observe the intercalation/de-intercalation processes of PF6− in graphite in real time. First, by measuring the change in the distance between graphene layers during intercalation, we found that PF6− intercalates in one of every three graphite layers and the intercalation speed is measured to be 2 µm·min−1. Second, graphite will wrinkle and suffer structural damages at high voltages, along with severe electrolyte decomposition on the surface. These findings provide useful information for further optimizing the capacity and the stability of graphite anode in DIBs.

  • Journal article
    Madabattula G, Wu B, Marinescu M, Offer Get al., 2020,

    How to design lithium ion capacitors: modelling, mass ratio ofelectrodes and pre-lithiation

    , Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

    Lithium ion capacitors (LICs) store energy using double layer capacitance at the positive electrode and intercalation at the negative electrode. LICs offer the optimum power and energy density with longer cycle life for applications requiring short pulses of high power. However, the effect of electrode balancing and pre-lithiation on usable energy is rarely studied. In this work, a set of guidelines for optimum design of LICs with activated carbon (AC) as positive electrode and lithium titanium oxide (LTO) as negative electrode was proposed. A physics-based model has been developed and used to study the relationship between usable energy at different effective C rates and the mass ratio of the electrodes. The model was validated against experimental data from literature. The model was then extended to analyze the need for pre-lithiation of LTO. The limits for pre-lithiation in LTO and use of negative polarization of the AC electrode to improve the cell capacity have been analyzed using the model. Furthermore, the model was used to relate the electrolyte depletion effects to poorer power performance in a cell with higher mass ratio. The open-source model can be re-parameterised for other LIC electrode combinations, and should be of interest to cell designers.

  • Journal article
    Ai W, Kraft L, Sturm J, Jossen A, Wu Bet al., 2020,

    Electrochemical thermal-mechanical modelling of stress inhomogeneity in lithium-ion pouch cells

    , Journal of The Electrochemical Society, Vol: 167, ISSN: 0013-4651

    Whilst extensive research has been conducted on the effects of temperature in lithium-ion batteries, mechanical effects have not received as much attention despite their importance. In this work, the stress response in electrode particles is investigated through a pseudo-2D model with mechanically coupled diffusion physics. This model can predict the voltage, temperature and thickness change for a lithium cobalt oxide-graphite pouch cell agreeing well with experimental results. Simulations show that the stress level is overestimated by up to 50% using the standard pseudo-2D model (without stress enhanced diffusion), and stresses can accelerate the diffusion in solid phases and increase the discharge cell capacity by 5.4%. The evolution of stresses inside electrode particles and the stress inhomogeneity through the battery electrode have been illustrated. The stress level is determined by the gradients of lithium concentration, and large stresses are generated at the electrode-separator interface when high C-rates are applied, e.g. fast charging. The results can explain the experimental results of particle fragmentation close to the separator and provide novel insights to understand the local aging behaviors of battery cells and to inform improved battery control algorithms for longer lifetimes.

  • Software
    Madabattula G, Wu B, Marinescu M, Offer Get al., 2019,

    1D Electrochemical Model for Lithium Ion Capacitors in Comsol

    Lithium ion capacitor is an electrochemical energy storage device with optimum energy density, power density and longer cycle life. A 1D-electrochemical model for activated carbon (AC)/ lithium titanium oxide (LTO) based lithium ion capacitor was built in COMSOL multiphyisics, v5.3a. The model was used to generate the data in an open-access paper: How to Design Lithium Ion Capacitor: Modelling, Mass Ratio of Electrodes and Pre-lithiation, Journal of The Electrochemical Society, 2020, 167. (http://jes.ecsdl.org/content/167/1/013527.abstract) The model can be used to optimize the mass ratio of electrodes and pre-lithiation level. It can be extended to study the capacity fade in the devices.

  • Journal article
    Chen X, Liu X, Ouyang M, Childs P, Brandon N, Wu Bet al., 2019,

    Electrospun composite nanofibre supercapacitors enhanced with electrochemically 3D printed current collectors

    , Journal of Energy Storage, Vol: 26, Pages: 100993-100993, ISSN: 2352-152X

    Carbonised electrospun nanofibres are attractive for supercapacitors due to their relatively high surface area, facile production routes and flexibility. With the addition of materials such as manganese oxide (MnO), the specific capacitance of the carbon nanofibres can be further improved through fast surface redox reactions, however this can reduce the electrical conductivity. In this work, electrochemical 3D printing is used as a novel means of improving electrical conductivity and the current collector-electrode interfacial resistance through the deposition of highly controlled layers of copper. Neat carbonised electrospun electrodes made with a 30 wt% manganese acetylacetonate (MnACAC) and polyacrylonitrile precursor solution have a hydrophobic nature preventing an even copper deposition. However, with an ethanol treatment, the nanofibre films can be made hydrophilic which enhances the copper deposition morphology to enable the formation of a percolating conductive network through the electrode. This has the impact of increasing electrode electronic conductivity by 360% from 10 S/m to 46 S/m and increasing specific capacitance 110% from 99 F/g to 208 F/g at 5 mV/s through increased utilisation of the pseudocapacitive active material. This novel approach thus provides a new route for performance enhancement of electrochemical devices using 3D printing, which opens new design possibilities.

  • Journal article
    Pang M-C, Hao Y, Marinescu M, Wang H, Chen M, Offer GJet al., 2019,

    Experimental and numerical analysis to identify the performance limiting mechanisms in solid-state lithium cells under pulse operating conditions.

    , Physical Chemistry Chemical Physics, Vol: 21, Pages: 22740-22755, ISSN: 1463-9076

    Solid-state lithium batteries could reduce the safety concern due to thermal runaway while improving the gravimetric and volumetric energy density beyond the existing practical limits of lithium-ion batteries. The successful commercialisation of solid-state lithium batteries depends on understanding and addressing the bottlenecks limiting the cell performance under realistic operational conditions such as dynamic current profiles of different pulse amplitudes. This study focuses on experimental analysis and continuum modelling of cell behaviour under pulse operating conditions, with most model parameters estimated from experimental measurements. By using a combined impedance and distribution of relaxation times analysis, we show that charge transfer at both interfaces occurs between the microseconds and milliseconds timescale. We also demonstrate that a simplified set of governing equations, rather than the conventional Poisson-Nernst-Planck equations, are sufficient to reproduce the experimentally observed behaviour during pulse discharge, pulse charging and dynamic pulse. Our simulation results suggest that solid diffusion in bulk LiCoO2 is the performance limiting mechanism under pulse operating conditions, with increasing voltage loss for lower states of charge. If bulk electrode forms the positive electrode, improvement in the ionic conductivity of the solid electrolyte beyond 10-4 S cm-1 yields marginal overall performance gains due to this solid diffusion limitation. Instead of further increasing the electrode thickness or improving the ionic conductivity on their own, we propose a holistic model-based approach to cell design, in order to achieve optimum performance for known operating conditions.

  • Book chapter
    Liu X, George C, Wang H, Wu Bet al., 2019,

    Novel inorganic composite materials for lithium‐ion batteries

    , Encyclopedia of Inorganic and Bioinorganic Chemistry, Publisher: Wiley

    Lithium‐ion batteries (LIBs) have revolutionized the way we interact with the world around us. This is in part due to their unrivaled energy density and stability relative to other energy storage chemistries such as lead‐acid, nickel–metal hydride, and nickel–cadmium batteries. Given the drive to reduce greenhouse gas emissions from road transport, LIBs have now transitioned from application in consumer electronics to be the most critical component for electric vehicles (EVs); however, improvements in energy and power density, cost reduction, and lifetime are still required. The key aspects of an LIB that define its performance are mainly the anode, cathode, and electrolyte, however development of the separator and current collectors are also key considerations. In the vast majority of commercially available LIBs, the anode consists mostly of graphite and the cathode mostly of layered transition metal oxides, with an organic electrolyte facilitating the lithium‐ion transport between the two electrodes. This article provides an overview of the state of the art in developing inorganic composite materials for LIBs and concludes by highlighting the current challenges as well as the potential opportunities in the field.

  • Journal article
    Zhao Y, Diaz LB, Patel Y, Zhang T, Offer GJet al., 2019,

    How to cool lithium ion batteries: optimising cell design using a thermally coupled model

    , Journal of The Electrochemical Society, Vol: 166, Pages: A2849-A2859, ISSN: 0013-4651

    Cooling electrical tabs of the cell instead of the lithium ion cell surfaces has shown to provide better thermal uniformity within the cell, but its ability to remove heat is limited by the heat transfer bottleneck between tab and electrode stack. A two-dimensional electro-thermal model was validated with custom made cells with different tab sizes and position and used to study how heat transfer for tab cooling could be increased. We show for the first time that the heat transfer bottleneck can be opened up with a single modification, increasing the thickness of the tabs, without affecting the electrode stack. A virtual large-capacity automotive cell (based upon the LG Chem E63 cell) was modelled to demonstrate that optimised tab cooling can be as effective in removing heat as surface cooling, while maintaining the benefit of better thermal, current and state-of-charge homogeneity. These findings will enable cell manufacturers to optimise cell design to allow wider introduction of tab cooling. This would enable the benefits of tab cooling, including higher useable capacity, higher power, and a longer lifetime to be possible in a wider range of applications.

  • Journal article
    Tomaszewska A, Chu Z, Feng X, O'Kane S, Liu X, Chen J, Ji C, Endler E, Li R, Liu L, Li Y, Zheng S, Vetterlein S, Gao M, Du J, Parkes M, Ouyang M, Marinescu M, Offer G, Wu Bet al., 2019,

    Lithium-ion battery fast charging: A review

    , eTransportation, Vol: 1, Pages: 1-28, ISSN: 2590-1168

    In the recent years, lithium-ion batteries have become the battery technology of choice for portable devices, electric vehicles and grid storage. While increasing numbers of car manufacturers are introducing electrified models into their offering, range anxiety and the length of time required to recharge the batteries are still a common concern. The high currents needed to accelerate the charging process have been known to reduce energy efficiency and cause accelerated capacity and power fade. Fast charging is a multiscale problem, therefore insights from atomic to system level are required to understand and improve fast charging performance. The present paper reviews the literature on the physical phenomena that limit battery charging speeds, the degradation mechanisms that commonly result from charging at high currents, and the approaches that have been proposed to address these issues. Special attention is paid to low temperature charging. Alternative fast charging protocols are presented and critically assessed. Safety implications are explored, including the potential influence of fast charging on thermal runaway characteristics. Finally, knowledge gaps are identified and recommendations are made for the direction of future research. The need to develop reliable in operando methods to detect lithium plating and mechanical degradation is highlighted. Robust model-based charging optimisation strategies are identified as key to enabling fast charging in all conditions. Thermal management strategies to both cool batteries during charging and preheat them in cold weather are acknowledged as critical, with a particular focus on techniques capable of achieving high speeds and good temperature homogeneities.

  • Journal article
    Hales A, Diaz LB, Marzook MW, Zhao Y, Patel Y, Offer Get al., 2019,

    The cell cooling coefficient: A standard to define heatrejection from lithium-ion batteries

    , Journal of The Electrochemical Society, Vol: 166, Pages: A2383-A2395, ISSN: 0013-4651

    Lithium-ion battery development is conventionally driven by energy and power density targets, yet the performance of a lithium-ion battery pack is often restricted by its heat rejection capabilities. It is therefore common to observe elevated cell temperatures and large internal thermal gradients which, given that impedance is a function of temperature, induce large current inhomogeneities and accelerate cell-level degradation. Battery thermal performance must be better quantified to resolve this limitation, but anisotropic thermal conductivity and uneven internal heat generation rates render conventional heat rejection measures, such as the Biot number, unsuitable. The Cell Cooling Coefficient (CCC) is introduced as a new metric which quantifies the rate of heat rejection. The CCC (units W.K−1) is constant for a given cell and thermal management method and is therefore ideal for comparing the thermal performance of different cell designs and form factors. By enhancing knowledge of pack-wide heat rejection, uptake of the CCC will also reduce the risk of thermal runaway. The CCC is presented as an essential tool to inform the cell down-selection process in the initial design phases, based solely on their thermal bottlenecks. This simple methodology has the potential to revolutionise the lithium-ion battery industry.

  • Journal article
    Bravo Diaz L, Hales A, Zhao Y, Marzook MW, Patel Y, Offer GJet al., 2019,

    Cell Heat Generation and Dissipation: From Experimentation to Application for Cell Design.

    , ECS Meeting Abstracts, Vol: MA2019-04, Pages: 185-185

    <jats:p> Lithium ion batteries (LIBs) are increasingly important in ensuring sustainable mobility and reliable energy supply, storing and managing energy from renewable sources [1]. Temperature is a critical factor in LIBs performance optimisation where large temperature deviations within the cell could lead to accelerated degradation and in extreme cases, thermal runaway. Thermal management has therefore become the focus of intensive research in an attempt to improve battery performance and lifespan [2-5]. Despite the growing research interest in this area, cell heat generation and heat dissipation pathways are not usually considered when designing a cell. This typically leads to cells with thermal bottlenecks prone to internal thermal gradients. With the goal of improving performance and lifetime, a two-dimensional electro-thermal model has been developed to simulate cell performance and internal states under complex thermal boundary conditions [6]. This model can be used to assess different cooling strategies and parameters such us tab position and dimensions can be optimised from the thermal performance perspective for a particular cell chemistry and geometry. </jats:p> <jats:p>In this study, a novel experimental procedure is employed to evaluate cell heat generation and dissipation for various operation conditions. The two-dimensional electro-thermal model was employed to assess the internal temperature distribution during the measurements and to verify the heat dissipation patterns observed during the experiments. As a result, a new metric, the Cell Cooling Coefficient (CCC) is proposed to evaluate the thermal pathways of a cell cooled via its tabs. <jats:list list-type="simple"> <jats:list-item> <jats:p>International Energy Agency. Tracking Clean Energy Progress 2017. 1–82 (2017). doi:10.1787/energy_tech-2014-en</jats:p> <

  • Journal article
    Chen B, Offer GJ, Wang H, 2019,

    Visualising and Characterising Zinc Ion Transport for Zinc Ion Batteries By Fluorescence Microscopy

    , ECS Meeting Abstracts, Vol: MA2019-04, Pages: 225-225

    <jats:p> Zinc has been regarded as a promising anode material for aqueous batteries in view of its advantages including high specific capacity, abundance and intrinsic safety. Aqueous zinc ion batteries have attracted growing attention as a potential alternative to lithium ion batteries, especially for medium and large-scale energy storage. Aqueous zinc ion batteries consist of a zinc anode and a zinc intercalating cathode in a zinc-salt-containing electrolyte and use zinc ions as charge carriers. The electrolyte transporting zinc ions between the anode and the cathode plays an essential role in determining battery performance and life. Visualising ion transport in the electrolytes of zinc ion batteries can give insights into electrolyte dynamics as well as battery processes. Yet, this is limited by the spatial-temporal resolution of the existing techniques. Moreover, most of the existing in-situ visualisation techniques are very expensive and not easily accessible. Fluorescence microscopy provides a powerful tool for probing tiny structures and tracking species in real time. It relies on fluorescence which occurs when molecules absorb light with a certain wavelength followed by the re-emission of light with a longer wavelength. These excitation and emission wavelengths are usually unique fingerprints of certain substances. Fluorescent sensors, with high sensitivity of fluorescence assays, have been widely exploited as a useful tool for species detection and mechanistic studies in biology and chemistry. In this study, a novel microfluidics-based fluorescence microscopy platform is developed for visualising and characterising zinc ion transport in the electrolytes of zinc ion batteries. The platform is calibrated by comparing the measurement results with the literature data. Key transport properties of zinc ions are quantified under different electrolyte conditions. The developed platform is demonstrated to be a simple and versatile tool for electrolyte charac

  • Journal article
    Madabattula G, Marinescu M, Wu B, Offer GJet al., 2019,

    Effect of Mass Ratio of Electrodes in Lithium Ion Capacitors: Insights from a Physics-Based Model

    , ECS Meeting Abstracts, Vol: MA2019-04, Pages: 193-193

    <jats:p> Lithium ion capacitors (LICs) store electrical energy in the form of a double layer at the high surface area positive electrode such as activated carbon (AC) and in the form of lithium intercalation at the negative electrode through materials such as lithium titanium oxide (LTO). Due to unequal specific capacities and different physical properties of the electrodes, the mass ratio of electrodes (AC/LTO) has to be optimized for the improved performance of LICs at high currents. In this work, we use a physics-based model and compare the model predictions with the experimental data to show the effect of electrode mass ratio and highlight the influence of electrolyte salt depletion/precipitation effects at different currents. Our results show that lower mass ratio of the electrodes (AC/LTO) is better for high power performance of LICs. </jats:p>

  • Conference paper
    Pang M-C, Hao Y, Wang H, Marinescu M, Chen M, Offer Get al., 2019,

    What is the rate-limiting mechanism in solid-state lithium cells at different pulse operating conditions?

    , 236th ECS Meeting
  • Conference paper
    Pang M-C, Hao Y, Wang H, Marinescu M, Chen M, Offer Get al., 2019,

    Experimental Parameterisation of the Continuum Models for Solid-state Lithium Batteries

    , 3rd Annual Oxford ECS Student Chapter Symposium
  • Journal article
    Yin C, Liu X, Wei J, Tan R, Zhou J, Ouyang M, Wang H, Cooper SJ, Wu B, George C, Wang Qet al., 2019,

    “All-in-Gel” design for supercapacitors towards solid-state energy devices with thermal and mechanical compliance

    , Journal of Materials Chemistry A, Vol: 7, Pages: 8826-8831, ISSN: 2050-7488

    Ionogels are semi-solid, ion conductive and mechanically compliant materials that hold promise for flexible, shape-conformable and all-solid-state energy storage devices. However, identifying facile routes for manufacturing ionogels into devices with highly resilient electrode/electrolyte interfaces remains a challenge. Here we present a novel all-in-gel supercapacitor consisting of an ionogel composite electrolyte and bucky gel electrodes processed using a one-step method. Compared with the mechanical properties and ionic conductivities of pure ionogels, our composite ionogels offer enhanced self-recovery (retaining 78% of mechanical robustness after 300 cycles at 60% strain) and a high ionic conductivity of 8.7 mS cm−1, which is attributed to the robust amorphous polymer phase that enables facile permeation of ionic liquids, facilitating effective diffusion of charge carriers. We show that development of a supercapacitor with these gel electrodes and electrolytes significantly improves the interfacial contact between electrodes and electrolyte, yielding an area specific capacitance of 43 mF cm−2 at a current density of 1.0 mA cm−2. Additionally, through this all-in-gel design a supercapacitor can achieve a capacitance between 22–81 mF cm−2 over a wide operating temperature range of −40 °C to 100 °C at a current density of 0.2 mA cm−2.

  • Journal article
    Campbell I, Gopalakrishnan K, Marinescu M, Torchio M, Offer G, Raimondo Det al., 2019,

    Optimising lithium-ion cell design for plug-in hybrid and battery electric vehicles

    , Journal of Energy Storage, Vol: 22, Pages: 228-238, ISSN: 2352-152X

    Increased driving range and enhanced fast charging capabilities are two immediate goals of transport electrification. However, these are of competing nature, leading to increased energy and power demand respectively from the on-board battery pack. By fine-tuning the number of layers versus active electrode material of a lithium ion pouch cell, tailored designs targeting either of these goals can be obtained. Achieving this trade-off through iterative empirical testing of layer choices is expensive and often produces sub-optimal designs. This paper presents a model-based methodology for determining the optimal number of layers, maximising usable energy whilst satisfying specific acceleration and fast charging targets. The proposed methodology accounts for the critical need to avoid lithium plating during fast charging and searches for the optimal layer configuration considering a range of thermal conditions. A numerical implementation of a cell model using a hybrid finite volume-spectral scheme is presented, wherein the model equations are suitably reformulated to directly accept power inputs, facilitating rapid and accurate searching of the layer design space. Electrode materials exhibiting high solid phase diffusion rates are highlighted as being equally as important for extended range as the development of new materials with higher inherent capacity. The proposed methodology is demonstrated for the common module design of a battery pack in a plug-in hybrid vehicle, thereby illustrating how the cost of derivative vehicle models can be reduced. To facilitate model based layer optimisation, the open-source toolbox, BOLD (Battery Optimal Layer Design) is provided.

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